There is at least one potentially good thing about genetically modified crops: In theory, substituting crops such as soybeans for versions modified to resist products like Monsanto’s Roundup could actually reduce the amount and toxicity of the herbicides farmers deployed. Unfortunately, exactly the opposite has happened, according to a study published this week.

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“We find clear evidence of increasing herbicide use by [genetically engineered] variety adopters over time for both soybeans and maize, a finding that we attribute in part to the emergence of glyphosate weed resistance,” Edward Perry, GianCarlo Moschini, and their colleagues write in Science Advances.

Glyphosate-resistant crops (glyphosate is the active ingredient in Roundup) were first introduced in 1996. Environmental friendliness was listed among their potential benefits, in part because glyphosate is less toxic to animals than other herbicides, and because farmers can get away with using much less herbicide overall. (Presumably, those crops also made a lot of sense to Monsanto’s bottom line.)

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The problem is, using just one pesticide could produce glyphosate-resistant weeds through natural selection, in the same way that widespread antibiotic use has created antibiotic-resistant bacteria. If that happened, farmers would most likely have to use increasingly more toxic herbicides to achieve the same results.

Using just one pesticide could produce glyphosate-resistant weeds through natural selection.

And, according to the researchers, that appears to be what’s happened. The team based their results on data collected between 1998 and 2011 by GfK Kynetec, a market research firm that surveys farmers about their seed and pesticide use. That data showed that, contrary to what some had expected (or hoped, at least) herbicide use has increased over time, and environmental impact has dropped only slightly, if at all.

“Since 1998, the most striking trend has been an increase in the use of glyphosate,” the team writes. “By 2011, glyphosate dominated the soybean herbicide market with just over 80% of total herbicide applied, and in maize, it accounted for nearly 40% of applied herbicide (a near 20-fold increase from 1998).”

While some of the increase in glyphosate use came at the expense of other herbicides, the team found that overall herbicide use in soybean fields has increased by about one-third since 2007, before which levels were fairly flat. On the other hand, because of the changing mix of herbicides, the environmental impact of soybean herbicides has stayed largely the same. For corn, farmers are using about the same amount of herbicides, but the environmental impact has dropped slightly since 1998.

Still, that’s not good news. The results, the authors argue, are consistent with the possibility that using Roundup has created Roundup-resistant weeds. That hypothesis gets an extra boost from the fact that the number of plots exclusively using glyphosate was on the rise until 2006, after which it declined, suggesting farmers are having a harder time killing weeds with Roundup.

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